Minnesota's measles outbreak was bad but could've been worse

Minnesota can't quite shake the measles this year, as the highly infectious and itchy disease is spreading one by one to unvaccinated people in the Twin Cities — mostly preschool-age children — presenting the risk of a broader outbreak.

The 59 infections so far make up the second-highest total of any U.S. state in 2024 and the second-highest annual tally in Minnesota in 35 years.

Public health officials said the total, including 12 severe cases requiring hospitalization, could have been worse if not for their rapid response. Epidemiology investigations linked 51 cases and identified 1,900 close contacts of infected people at risk of contracting and spreading measles, including 240 people who didn't have immunity as a result of vaccinations or prior infections.

Those unprotected people were asked to stay away from others until their infection risks were established, and their compliance over some difficult days and weeks made a difference, said Jayne Griffith, a senior epidemiologist for infectious diseases at the Minnesota Department of Health.

"When it comes to measles, once we have conversations with families, and they certainly understand how sick their child has been, they do want to cooperate," she said. "We hear from parents all the time that they don't want someone else's child to be that sick."

The measles virus is one of the most infectious agents on the planet, capable of lingering in the air and spreading to others long after sick people have left. Known by its characteristic head-to-toe rash, measles also causes high fevers, cold symptoms and breathing problems. A two-dose vaccine is 97% effective at preventing measles, but the infectious disease is making a comeback in the U.S. as parents decline the shots for their children.

Minnesota had the sixth-worst rate of children vaccinated in time for kindergarten at the start of the 2022-2023 school year, a recent federal report showed . A decade ago, the state's measles vaccination rate of 94% was among the best, but it has since declined to 87%.

Sunday marked about 12 days since the last identified infection, but state health officials don't feel relief until 21 days — which is how long it can take in extreme cases for exposure to the measles to produce symptoms. Measles outbreaks aren't considered over until two incubation cycles, or 42 days, have passed, so the threat will remain in Minnesota until almost Halloween.

The current situation is similar to Minnesota's 2017 outbreak of 75 cases, which also occurred mostly among Somali immigrants, who have been fearful that the measles vaccine increases risks of childhood autism, despite substantial research to the contrary.

Yet it has presented different challenges than the 2017 outbreak that centered on day care facilities and toddlers, or even this year's outbreak in Chicago that emerged in a temporary shelter for migrants. Those weren't discovered until locally infected people had already passed measles on to more people, who had passed it on to more people.

"That's when its like, 'uh oh, there's a lot of people that got exposed that we don't know about,'" Griffith said. "With [the 2024 outbreak], we've been able to stay on top of things a little bit more quickly."

This year's Minnesota outbreak started in May with three related children who were infected during travels to a country where measles is common. Health officials identified close contacts at risk who hadn't been vaccinated and asked them to stay away from others, but the virus slipped through. People carrying the virus can be infectious for four days before rashes emerge, said Erica Bagstad, an infectious disease epidemiology supervisor for Hennepin County Public Health, which has worked closely with people exposed to measles during their stay-at-home periods.

"So you might have a little runny nose and a tiny cough and you wouldn't think twice about going out in public" and unknowingly spreading measles, she said.

Two measles cases emerged in late June, followed by three in July among children from different metro counties who had no apparent links.

State leaders predicted that those infections traced back to the May travel cases, but they lacked proof. Worse yet, too much time had passed for the children infected in May to have passed measles to the late summer cases.

Establishing that link became a priority. The lack of a connection could suggest that measles had established itself in Minnesota, spreading from person to person, and becoming more difficult to track and contain.

Testing at Minnesota's public health lab found that the cases stemmed from the same D8 strain of measles virus, but more proof was needed. The coronavirus that causes COVID-19 changes all the time, so epidemiologists can track how it spreads from person to person based on its genetic mutations. But the measles virus doesn't change much, so it can't be tracked in the same way.

Interviews and re-interviews of people affected by the outbreak eventually found the link. Someone who had been in contact with people from the May and late June clusters had been sick with measles-like symptoms in early June but never sought treatment at a clinic or hospital.

State and local public health workers tried to build a wall in late summer around the latest measles cases. They asked unvaccinated close contacts to stay at home and called them or their parents frequently to see if symptoms emerged. When contacts got sick, workers called ahead to clinics and hospitals to admit them without risking exposure to other patients.

Still, the disease spread. Almost one case per day was identified in August, though mostly within the families and close circles of infected people. The 51 cases so far in the outbreak include 28 preschool children and 23 people older than 5, including one adult.

A state map of the outbreak shows how five children were infected in three day care facilities, and how the disease then spread to contacts at home. One person was infected at a health care facility, while another person with measles in late August passed the disease on to six others in the same household.

At least one case involved a child who exposed others at a Minneapolis Public Schools elementary school. Griffith said no cases have emerged from that possible school exposure, but its too early to know for sure.

The vaccine hesitance of Somali families has been a problem for years, he said, even though the evidence against the autism fear has strengthened. If there was a link, then the decline in vaccinations among Somali children should have coincided with a decline in autism. That hasn't happened.

What troubles Osterholm is the spread of vaccine hesitancy to other demographic groups since the pandemic. Some were so upset by the mandates and pressures to get vaccinated for COVID-19 that they have declined other vaccines with established safety and effectiveness records.

"They hit a critical point where they just took off with thousands of cases," Osterholm said. "I worry we may be close to that point here."